Page 176 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

148 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

3.5.2 Ultrasound-based methods

Ultrasound-based methods for liquid metals rely on the transmission of ultrasonic
bursts into the melt and on the detection of the echo that is reflected by microparticles
or bubble interfaces. By the knowledge of the sound velocity in the melt, the distance
between the ultrasound transmitter and the scattering object can be determined (time-
of-flight method). This principle is used by UDV for measuring the spatially resolved
velocity information in nearly real time and by UTTT, which detects the position and
the diameter of bubbles in the melt. Ultrasonic methods are noninvasive, but not con-
tactless since a continuous acoustic transmission path from the ultrasonic transducer to
the fluid under investigation is required.
The measuring principle of UDV is shown in Fig. 3.5.1. Ultrasound bursts emitted
from an ultrasonic transducer propagate into the fluid along the measuring line coin-
ciding with the extended transducer normal. A part of the ultrasonic pulse energy is
scattered by microparticles suspended in the liquid. The reflected signal is received by
the same transducer and contains the entire information of the velocity profile along
the ultrasonic beam. The knowledge of the sound velocity of the liquid allows deter-
mining the position of the scattering particles on the measuring line from the measured
time delay between the burst emission and the reception of the respective echo signals.
The movement of the scattering particles inside the measuring volume will result in a

Fig. 3.5.1 Measurement principle of ultrasound Doppler velocimetry.